[clang][dataflow] HTML logger: Mark iterations that have converged. (#68204)

I've eliminated the `logText("Block converged")` call entirely because

a) These logs are associated with an individual `CFGElement`, while
convergence
   should be associated with a block, and

b) The log message was being associated with the wrong block:
`recordState()`
dumps all of the pending log messages, but `blockConverged()` is called
after
   the last `recordState()` call for a given block, so that the "Block
converged" log message was being associated with the first element of
the
   _next_ block to be processed.

Example:


![image](https://github.com/llvm/llvm-project/assets/29098113/6a19095c-2dbb-4771-9485-e8e45c9d26fb)